Gallium nitride-based (Group III–V) semiconductor, which has a relatively large bandgap, is one of the prospective materials suitable for the short wavelength light emitting devices used in optical information processing units handling the increasing amount of information contents. In such light emitting devices as a diode device or a laser device, a PN junction is the essential structure, where the carriers are recombined at the vicinity of the junction and the light is emitted. As is well known, it is not easy to provide a low resistivity nitride semiconductor because, in the p-type nitride semiconductor doped with magnesium, Mg, or other acceptor, the activation rate the acceptor is significantly lower relative to doner.
The p-type nitride semiconductor exhibits a high resistivity value even when it is restored to room temperature after growth. In order to obtain a low resistivity, it has been a normal practice to apply a post annealing or other thermal treatment on a p-type nitride semiconductor to dissociate the hydrogen of a complex formed of magnesium and hydrogen from magnesium. Research activities are being made to provide a low resistivity p-type nitride semiconductor without applying the post annealing. If it turns out to be successful, it will bring about an advantage also for an improved productivity in such devices.
In the US Patent Publication 5,932,896 (Japanese Patent Laid-open No. 135575/1998), for example, a method for manufacturing a p-type nitride semiconductor without applying the post annealing is disclosed.
The method disclosed in the above Publication uses a Metal-Organic Chemical Vapor Deposition (MOCVD) process to grow a p-type nitride semiconductor on a sapphire substrate; introducing an organic magnesium compound containing such Group III source as trimethylgallium (TMG), such nitrogen source as ammonia (NH3) and p-type dopant on the substrate of 1100° C. using a nitrogen carrier gas containing hydrogen gas at a 0.8%–20% concentration in the capacity percent. In this way, formation of a magnesium-hydrogen complex is blocked and a p-type nitride semiconductor that exhibits a low resistivity during growth stage is provided. It also discloses a cooling process, where the temperature is decreased to 350° C. in an atmosphere of nitrogen gas containing ammonia by approximately 32% in the capacity percent, and then supply of ammonia is suspended, and the temperature is lowered to room temperature.
The above describe conventional method for manufacturing a p-type nitride semiconductor eliminating the post annealing, however, has following problems. Namely, as the inventor of the above described U.S. Pat. No. 5,932,896 and other writer taught in a thesis (Applied Physics Letters, vol. 72, (1998), p. 1748), the activation rate of magnesium significantly deteriorates if the hydrogen concentration increased merely from 2.4% to 3.7% during crystal growth process, which means that a p-type nitride semiconductor is obtainable only when it is grown in a very low hydrogen concentration. What is more, if a p-type nitride semiconductor is grown in a low hydrogen concentration the surface migration turns out to be insufficient, and certain specific atoms are not disposed at respective optimum points on the surface, making it difficult to obtain a good crystal.